The present invention relates to television anti-copy protection processes which hamper or inhibit recording as, for example, on a video cassette recorder (VCR) or personal video recorder (PVR). In the field of anti-copy protection signals, negative going pulses are paired with positive going pulses. For example, pseudo sync and automatic gain control (AGC) pulses and or sync pulses and AGC pulses constitute signals that cause a reaction in AGC systems in a VCR, or are detected by a reading system in a compliant device such as a PVR, digital recorder, analog to digital convertor. In general, a video anti-copy protection process (e.g., copy protected video signal or a video signal including one or more copy protection signal such as pseudo sync/AGC pulses) is playable on a television (TV) display while generally causing darkening, color distortion, tearing, loss in at least a portion of a program video signal, or video gain effects in a recorded copy.
However, such signals have been circumvented by circumvention devices, commonly referred to as “black box” devices, which remove or attenuate the effects of the anti-copy protection pulses, while passing through the (active field) program video intact. Such “black boxes” generally replace the negative (e.g., pseudo sync) and or positive going pulses (e.g. AGC pulses/signals) with a blanking level or newly regenerated sync pulses free of pseudo sync/AGC signals.
A copy protection process disclosed in U.S. Pat. No. 4,631,603, John O. Ryan, December 1986, assigned to Macrovision Corporation, Santa Clara, Calif., incorporated by reference, is well known to have placed pseudo sync and AGC pulses in specific television (TV) lines for pre-recorded video home systems (VHS) tape and digital video disc (DVD) playback devices. These pseudo sync/AGC pulses inserted in a program video signal prohibit recording by affecting the AGC system of a recorder, while allowing (substantially) normal display of the program video signal. However, the makers of the circumvention devices have observed the locations of the added pulses in the vertical blanking interval (VB1), and accordingly have generated a timing pulse to blank out or to modify at least some of the copy protection signals.
Several of the newer circumvention devices do away with conventional timing circuits and rely on microprocessors to provide the blanking pulses. Other circumvention devices utilize traditional timing circuits (e.g., retriggerable one shot) to locate the copy protection pulses.
Also, in the field of copy protection, the anti-copy protection signals are usually hidden in a portion of the vertical blanking interval, which generally cause a small scanning error in the display device. But because most displays overscan the video information, which results in a cropped picture, the scanning error is generally not noticeable. However, with some displays with less overscan, a small but perceivable geometric distortion may be observed when certain anti-copy protection signals are applied in the vertical blanking interval.